VOGONS


First post, by abrunetto

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Hi folks, I hope you are doing well.

I just bought an IBM PS/2 model 30 8086 which didn't work. It had a short between -12V power rail and GND. I figured out that the C99 tantalum capacitor was burnt, and after replacing it the computer came back to life.

However there is still an issue with the MCGA on-board display adapter. I just stripped the blind pin from a standard VGA cable in order to use a regular VGA monitor (Samsung 2043NWX). I have display but it is half-cutted and splitted. I attached an image.

I didn't find C99 capacitor on the motherboard schematics, but it seems to be for filtering the -12V power rails. The whole -12V rail comes from the motherboard PSU P3 connector and it is connected to the ISA expansion slot pin B7 and B8, to the MC1488 RS-232 driver and finally to a ferrite followed by C99 to ground.

Surprisingly, next to C99 it is R15 and, according to schematics, it should be connected to +5V rail. However it is actually connected to -12V power rail. Certainly the schematics are wrong or they are a different version.

Anyway, the point is R15 is part of the input circuit to IREF which is on IMS G171 IC which provides MCGA graphics.

I didn't replace the capacitor (I don't have a spare). Could that missing cap be the responsible of this behaviour?

Just to let you know I tried with a VGA ISA card and it worked fine.

Any help will be highly appreciate.

Thank you very much!
Regards,
Ariel

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Reply 1 of 8, by bakemono

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The fact that characters show repeating data and there is a fault detectable by software (video memory test?) makes me think that the problem is with the video memory address bus (A3 line?)

yet another retro game on itch: https://90soft90.itch.io/super-wild-war-22

Reply 2 of 8, by abrunetto

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bakemono wrote on 2022-08-17, 21:06:

The fact that characters show repeating data and there is a fault detectable by software (video memory test?) makes me think that the problem is with the video memory address bus (A3 line?)

Thank you for your answer. Why do you think A3 line is involved?. Just to let you know the same thing happened even with the characters on the "pc and diskette boot screen". Attached you will find a screenshot.

Moreover first time I turned it on (after remove the C99 capacitor) the display was worst,

with a different character in every displayed character on the screen. It shows different characters.

Since the floppy drive is dead, also the HDD, I have to install an xtide like in order to run software tests.

Best Regards,
Ariel

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  • 20220817_151558.jpg
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    Boot screen with character display issue.
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Reply 3 of 8, by mkarcher

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bakemono wrote on 2022-08-17, 21:06:

The fact that characters show repeating data and there is a fault detectable by software (video memory test?) makes me think that the problem is with the video memory address bus (A3 line?)

Good idea. Font handling on the MCGA is quite special, though. The processor copies the font data from the BIOS ROM into standard video RAM, and then invokes some hardware magic that copies the contents of a special part of the standard video RAM (which is dynamic VRAM) into the font SRAM. The processor has no way to verify the contents of the font SRAM, so a problem in font SRAM addressing can't be the sole failure if the fault is detectable by software. Apart from this, CGA3 (pin 7 of the font SRAM, pin 27 of U11) would indeed be a primary suspect.

When transferring the font from VRAM to SRAM, the VRAM addresses are generated by U11 on VMA0 to VMA7. Row/column multiplexing is handled inside U11, so these lines carry both the row and the column addresses. On the other hand, when the processor accesses video memory, VMA0 to VMA7 are driven by the multiplexers U49/U50. As I described in Re: 8-bit guy gets ultra-rare NOS IBM (prototype?) from Computer Reset then proceeds to obliterate it , font upload puts CGA3 also on "A12" of the video memory, which is on VMA4 during row addressing, whereas VMA4 contains A4 during column addressing. As the text screens look mostly right, the VMA4 line is obviously not shorted to ground/Vcc, because in that case, we would see something erratic on 8 out of 16 characters. If A12 on the processor side was broken, the lower half of the fonts wouldn't get filled in video RAM, but still get read by the font transfer process, so half of the characters would be garbage, not repeating the other half.

What might possibly cause the issue: Broken video memory ignoring row address 4, but respecting column address 4. Very unlikely fault, and next-to-impossible fault on both VRAMs at the same time. Another possible cause: U50 driving VMA4 low when it shouldn't during a font transfer. For scan-out and font-transfer, column address bits are not used; a whole row is transferred to the VRAM shift registers. If VMA4 is stuck low only when U11 drives the VMA (hypothesis: U50 roguely drives it to ground instead of being disabled), but works while U50 passes column addresses, this symptom might be explainable.

abrunetto wrote on 2022-08-17, 18:31:

Anyway, the point is R15 is part of the input circuit to IREF which is on IMS G171 IC which provides MCGA graphics.

I checked the schematic you posted: +5V is appropriate at that point. It won't work with -12V. There are alternate IREF circuits that require -12V, so maybe IBM changed the circuit. As you see, the possible reasons I discussed for the character display failure are strictly on video RAM stuff, which is clearly separated from the G171 IREF supply, I don't think the missing cap may cause the issue. It might cause vertical banding ("jail bars") though, if -12V is used in the IREF circuit.

Reply 4 of 8, by mkarcher

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abrunetto wrote on 2022-08-17, 22:12:

Since the floppy drive is dead, also the HDD, I have to install an xtide like in order to run software tests.

I made a thread about fixing one of the PS/2 floppy drives, turned out I re-discovered that the electrolytics go bad in those drives. See Let's fix some PS/2 floppy drives! .

Reply 5 of 8, by abrunetto

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mkarcher wrote on 2022-08-17, 22:25:

Good idea. Font handling on the MCGA is quite special, though. The processor copies the font data from the BIOS ROM into standard video RAM, and then invokes some hardware magic that copies the contents of a special part of the standard video RAM (which is dynamic VRAM) into the font SRAM. The processor has no way to verify the contents of the font SRAM, so a problem in font SRAM addressing can't be the sole failure if the fault is detectable by software. Apart from this, CGA3 (pin 7 of the font SRAM, pin 27 of U11) would indeed be a primary suspect.

Thank you for such a detailed answer which has lead me to read the technical ref manual. I will come back soon.

Regards,
Ariel

Reply 6 of 8, by abrunetto

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Hi guys!!

I have an update. Today I switched to graphics mode through ROM BASIC with screen 2 and screen 3 command. Attached you will find the results. This time the characters looks good but the screen is duplicated and with some colored pixels in the "border".
Regards,
Ariel

Reply 7 of 8, by abrunetto

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Here the attachments.

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  • 20220818_111234.jpg
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    20220818_111234.jpg
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    File comment
    screen 2
    File license
    Public domain
  • 20220818_111117.jpg
    Filename
    20220818_111117.jpg
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    474.8 KiB
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    111 views
    File comment
    screen 1
    File license
    Public domain

Reply 8 of 8, by abrunetto

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I switched back to screen 0 from screen 1 and tje results is even worse.

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